Writing isn't brain surgery, an editor at a New York publishing house once remarked to me. If you can teach a medical student to operate on a brain, you might think you could teach any reasonably literate person to write a novel; it's not that difficult. But I've read the books that come out of the writing courses that proliferate across America - they arrive by the dozen - and when I reach the last page they invariably fail the "so what?" test.

None the less, write what you know, the creative-writing teachers declaim, and the students, who mostly know what it's like to be a student (young, horny, inexperienced, strapped for cash, with ambitions to write a novel), write about that. The dictum serves to enshrine a type of writing - introspective, autobiographical, built on the small incidents and poignant reversals of daily life - that has become the gold standard of American fiction.

When I first encountered the phrase (as a student: young, horny, etc), it struck me as rather useless. How could "write what you know" account for the writers who mattered most to me when I was growing up? I wasn't reading John Updike or Saul Bellow; I was reading JRR Tolkien and Mary Renault, writers who hardly seemed limited to personal experiences and the people they had slept with.

Tolkien had never seen a dragon and Renault had never met Alexander the Great, yet they transcribed these fascinations as convincingly as Updike wrote about adultery and office intrigues. (To be sure, Renault began by writing realistic, sexually charged novels set in post-war Britain, but for better or worse, Tolkien never deigned to give us an exposé of the secret lives of Oxford dons.)

Still, "write what you know" commanded such influence that I couldn't ignore it completely. I made my peace with it by turning it backwards: not write what you know, but know what you write. If you write about a world before, after, or other than this one, enter that world completely. Search it to find your deepest longings and most terrible fears. Let imagination carry you as far as it may, as long as you recount the voyage with excitement and wonder.

But this is the most important rule: write the book you most long to read. Writing a first novel takes so much effort, with such little promise of result or reward, that it must necessarily be a labour of love bordering on madness. So it was with my first novel, Roman Blood. I returned from my first visit to Rome with my imagination on fire. Having become addicted to crime fiction via Conan Doyle, what I most wanted to read was a book that in 1989 seemed not to exist: a murder mystery set in ancient Rome.

I settled for the closest thing I could find, the Penguin edition of Cicero's Murder Trials, and found in the case of Sextus Roscius, accused of murdering his father, the seeds of the novel I was craving. But to read that novel, I would have to write it myself. So I did. I suspect this is how first novels are most often (or at least most successfully) sparked: a reader experiences an overwhelming craving for a book that does not yet exist. Whether the story draws on the first-time author's own life and experiences or not doesn't matter, so long as it conveys a truth that transcends the cramped tenets of mere realism.

· Steven Saylor's Honour the Dead is published by Constable Robinson at £7.99